In the field of cutting tool inserts and wear-resistant tools, collectively referred to as "tools" herein, an attempt has generally been made to substitute a ceramic tool or insert for a high-speed steel tool. A cemented carbide tool or a coated tool, and an Al.sub.2 O.sub.3 -based tool were first developed in the 1950s. However, these early tools were not brought into practice at the outset since their toughness was too small. Thereafter an improvement was made in order to increase the toughness of the tool by adding TiC, ZrO.sub.2 (Japanese Patent Publication No. 59-6274 (1984)), SiC whiskers (Japanese Patent Laying-Open No. 61-274803 (1986)), whereby the tool could be practically applied to finish cutting of cast iron or the like. However, it was impossible to use the tool for rough cutting and intermittent cutting since its toughness was still insufficient resulting in an inferior reliability. On the other hand, a silicon nitride sintered body can be widely applied to rough and/or finish cutting, wet cutting etc. of cast iron with high reliability since a silicon nitride sintered body has a higher toughness compared with an Al.sub.2 O.sub.3 sintered body and a high thermal shock resistance with a small thermal expansion coefficient. Japanese Patent Publication Nos. 6-19367 (1986) and 62-13430 (1987) have proposed tools which are obtained by coating the surfaces of silicon nitride sintered bodies with a Ti compound or Al.sub.2 O.sub.3 thereby improving the wear resistance of the silicon nitride sintered body and provide it with excellent characteristics.
As to such a silicon nitride sintered body, extensive studies have been made in relation to a sintering assistant, for the purpose of improving the strength, since silicon nitride is a highly covalent compound which has a large ratio of grain boundary energy to surface energy as compared to ionic and metallic crystals which large ratio causes an extremely slow self-diffusion, whereby silicon nitride is hardly sintered at a high temperature due to decomposition and evaporation. Thus, there has been developed a technique of forming a glass phase having a low melting point with a sintering assistant such as MgO, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, ZrO.sub.2, AlN, CaO, CeO.sub.2, SiO.sub.2 or the like for obtaining a dense sintered body by liquid phase sintering. In the history of the development of such sintering assistants, the development of Y.sub.2 O.sub.3 as a sintering assistant was extremely important because not only the sintered body was densified but also silicon nitride crystal grains were developed in columnar shapes to improve the strength of the silicon nitride sintered body by a theory of fiber reinforcement (see Japanese Patent Publication No. 48-7486 (1973)). In this sintering mechanism, .alpha.-Si.sub.3 N.sub.4 is dissolved in a molten glass phase which has been formed at a high temperature, deposited as .beta.-Si.sub.3 N.sub.4 (or .beta.'-SIALON) by a dissolution and/or redeposition phenomenon, and grown in a C-axis direction of a hexagonal system in the presence of Y.sub.2 O.sub.3, to form columnar silicon nitride particles.
Due to such technical development, it has been made possible to improve the strength of silicon nitride, which is now practically applied to make ceramic tools including tool inserts capable of carrying out rough cutting, intermittent cutting and rough milling of cast iron.
Such development of the sintering assistant and employment of gaseous nitrogen pressure sintering have also enabled sintering of silicon nitride in a pressurized nitrogen atmosphere. Previously, the sintering generally took place only by a hot pressing method. Further, it has been made possible to form a sintered body having a complicated configuration, in a near "net" shape, i.e., in a shape substantially close to that of the final product.
However, although it has been made possible to work a sintered body into a near final shape by the aforementioned technical development, there has not yet been developed a product of a silicon nitride tool or insert which is directly formed with a sintered surface capable of functioning as a cutting edge similarly to a cemented carbide or cermet tool formed with a directly coated sintered surface. Rather, a silicon nitride tool or insert requires grinding to obtain its finished shape under the present circumstances. Thus, the strength of the sintered body is reduced by cracks resulting from flaws which are caused during grinding, and the manufacturing cost is increased since silicon nitride is extremely hard to work making the grinding operation difficult. FIGS. 1A and 1B are microphotographs taken by a scanning electron microscope (hereinafter referred to as "SEM"), showing the state of a ground surface of a base material.
Although an attempt has been made to mold silicon nitride into a desired shape and finish the same without any grinding or through an after treatment with a low-cost method other than grinding, the so formed product deteriorated in strength and had a reduced wear resistance due to the presence of abnormal phases formed on the surface of the sintered body. Such abnormal phases appearing on the surface of the sintered body may be formed by sublimation and/or decomposition of Si.sub.3 N.sub.4, scattering of grain boundary glass phases and the like. It has been proposed to solve this problem by oxidizing the surface of a silicon nitride sintered body and forming a film which is mainly composed of silicon dioxide for covering abnormal phases appearing on the surface of the sintered body, thereby drawing out characteristics present in the silicon nitride sintered body itself (Japanese Patent Laying-Open No. 2-164773 (1990)). When such a sintered body is used as a tool or tool insert in practice, however, depositions are easily caused between the tool and a workpiece due to silicate glass phases having a low melting point which are formed on the surface of the sintered body, whereby the wear resistance is reduced. There has also been proposed a method of sintering a vessel of a specific material containing a pressed body of silicon nitride made of silicon nitride group powder held within a graphite crucible in a state buried in the silicon nitride group powder thereby obtaining a sintered material which can be applied to practical use in the sintered state (Japanese Patent Publication No. 62-30152 (1987)). On the surface of the so formed sintered body, however, freely grown columnar crystal grains of silicon nitride are formed as shown in FIGS. 2A and 2B, similarly to the case of an ordinary normal pressure sintering method. The columnar silicon nitride crystal grains, which are of Si.sub.3 N.sub.4 (or .beta.'-SIALON), are intertwined in the sintered body to improve its strength by the mechanism of fiber reinforcement as described above, thereby enabling the tool or the insert to perform cutting work, such as rough milling or intermittent cutting of cast iron. This phenomenon is specific to a silicon nitride sintered body which is improved in strength, and it is not observed in a sintered surface of a cermet or cemented carbide tool.
The crystal grains of .beta.-Si.sub.3 N.sub.4 or .beta.'-SIALON freely grown on the surface of the sintered body are instable due to the free or random growth, and merely fixed in place by glass phases mainly composed of a sintering aid located at end portions of the columnar crystal grains. Therefore, the crystal grains easily drop out when external stress is applied to the sintered body, whereby the surface roughness of the sintered body is increased due to clearances defined between the columnar particles. When a sintered body having such a surface is used as a tool or insert, therefore, its wear resistance is reduced and its strength is deteriorated by the columnar crystal grains falling out of the body surface.
In the case of a surface-coated silicon nitride sintered body which is used as a tool or insert, external stress is applied to a film covering the surface of its base material, to easily cause the above mentioned dropping phenomenon, while its surface roughness is increased due to clearance defined between columnar particles, and portions of the coating film become irregular to cause local grain growth, leading to the formation of a massive coating film as shown in FIGS. 5A and 5B.
When a sintered body having such surface phases is used as a tool, therefore, the cutting resistance is disadvantageously decreased and the coating film easily peels off to damage the effect of the coating. Such a problem is specific to a tool of a surface-coated silicon nitride sintered body which is improved in strength by columnar .beta.-Si.sub.3 N.sub.4 (or .beta.'-SIALON), and is not observed in a film coated on a cemented carbide tool. Further, in a silicon nitride sintered body, this problem has not been caused since a ground sintered body has been coated in general.